Isotopes

Key Definition Isotopes are atoms of the same element that share the same proton number but contain different numbers of neutrons.

This means they sit in the same place on the periodic table yet have different mass numbers.

The notation ˣᵧA encodes both values: the upper-left x is the mass number (protons + neutrons), while the lower-left y is the proton number (protons only). Subtracting y from x gives the neutron count, and in a neutral atom the electron count equals the proton number.

Because isotopes of an element share identical electron configurations, they undergo identical chemical reactions and form identical bonds. Neutrons are nuclear particles and do not influence bonding. However, additional neutrons add mass without changing atomic volume, so isotopes differ in mass and density.

The syllabus restricts examinable physical differences to these two. The clearest exam strategy is to link “same chemistry” to electrons and “different physics” to neutrons.